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LM25037EVAL: LM25037 SoftStart

watchaphong Aoopree1
Prodigy 80 points
Part Number: LM25037EVAL
Other Parts Discussed in Thread: LM25037

Hi TI Team
       I have got your LM25037EVAL board. after verify function i have some question about softstart circuit.
        In schematic use SS Cap = 0.68uF. So T3 should be around 27mS. but after test gate drive goes to maximum duty as 0.5mS.

Best regards

Watchaphong

  • 0
    TI__Expert 6080 points

    Hello there,

    The duty cycle of the controller is set by the selection of dead time and not the SS cap. 

    May I refer you to the 8.2.2.1 section of the datasheet for this: equation 11 would help you calculate the RT2 resistor for a dead time of 200ns and 95% of the max duty cycle.

    Regards,

    Sonal

  • 0 in reply to Sonal Singh
    Prodigy 80 points

    Hi Sonal
           Yes,I understand duty cycle is set by RT2. But the mypoint is i need to increase output rise time. and i have more question as below.

    1. Is it possible LM25037 for pushpull step up 12V to 400V. 
    2. Is it possible to increase output turn on rise time more than 100mS. by set CAP SS.

    Best regards

    Watchaphong

  • 0 in reply to watchaphong Aoopree1
    TI__Expert 6080 points

    Hello there,

    To answer your questions:

    1. No, the absolute maximum on the Vin pin of the controller to GND is 76V. The controller will not be able to survive 400V.
    2. Yes, you can increase the rise time of the output by increasing the cap value on the SS pin, a bigger cap would require longer to charge via the internal 100uA source thereby slowing the soft-start process.

    Regards,

    Sonal